1. Field of the Invention
The present invention relates generally to a switching power supply that feeds isolated DC electric power from a DC voltage source to a load via a transformer and a semiconductor switch and more particularly relates to a switching power supply that includes a changeover circuit for achieving power saving operations.
2. Description of the Prior Art
FIG. 3 is a circuit diagram of a conventional switching power supply which includes a mode changeover circuit. Referring to FIG. 3, a capacitor C0 is connected to the output of a rectifier Rect 1. A series circuit composed of a primary winding N1 of a transformer TR1 and a semiconductor switch Q1, such as a MOSFET, is connected in parallel with the capacitor C0. A capacitor C1 is connected to a first secondary winding N21 of the transformer TR1 through a series diode D1. A capacitor C2 is connected to a second secondary winding N22 of the transformer TR1 via a diode D2. A main load is connected in parallel to the capacitor C1 at terminals 300, 302. A microcomputer load is connected in parallel to the capacitor C2 at terminals 304, 302. A capacitor C3 is connected to a tertiary winding N3 of the transformer TR1 via a diode D3. The capacitor C3 is connected to a quaternary winding N4 of the transformer TR1 through a diode D4 and a transistor T4.
A voltage detector circuit is connected in parallel to the capacitor C1. The voltage detector circuit includes resistors R2 through R5, a photo-coupler PC1, a shunt regulator IC3 and a transistor T3. A switching circuit composed of transistor T1 driven by transistor T2 is connected between the diodes D1 and D2. A terminal 306 receives a power saving signal which is applied to transistors T2, T3 and a photo-coupler PC2. The switching power supply further includes a first control circuit IC1, which is operational during ordinary operation and a second control circuit IC2 which is operational during a power saving operation. The outputs of both IC1 and IC2 are connected to the gate of the switch Q1. The output of the photo-couplers PC1, PC2, are connected to the respective input terminals of the control circuits IC1 and IC2. Usually, the control circuits IC1, IC2 and the switch Q1 are integrally mounted in a single package as a power IC 308, illustratively surrounded by the broken lines in FIG. 3.
The circuit of FIG. 3 rectifies an AC voltage applied to terminals 310, 312 in the rectifier Rect 1 and applies the resulting DC voltage to the capacitor C0. As a result of the switching of the switch Q1 to an on state, the electric power stored in the capacitor C0 is transferred to the primary winding N1. By switching the switch Q1 to an off state, the energy stored in the transformer TR1 is fed to the capacitors C1, C2 via the first secondary winding N21 and the second secondary winding N22 and to the loads connected in parallel to the respective capacitors C1 and C2, thus providing an isolated DC voltage to the board. The circuit of FIG. 3 modulates the on-off ratio of the switch Q1 to regulate the output voltage, that is the voltage VC1 of the capacitor C1. The circuit, including the resistors R2 through R6, the photo-coupler PC1 and the shunt regulator IC3, detects the isolated output voltage and adjusts the on-off ratio of the switch Q1 via the control circuits IC1 and IC2 so that the detected voltage may be maintained at a certain value.
The voltage detector circuit works in the following manner. The shunt regulator IC3 feeds a current through the resistor R2 to maintain the voltage at the control terminal at a constant level. Here, the control terminal voltage is a voltage determined by the resistance values of resistors R3 through R5, as follows: VC1.multidot.[R4.multidot.R5/(R3.multidot.R4+R4.multidot.R5+R5.multidot.R3 )]. As a result, a constant current flows to the input terminal of the photo-coupler PC1. In response, a proportional constant current determined by the amplification function of the photo-coupler PC1 flows from the output terminal of the photo-coupler PC1. As a result, the voltage of the resistor R6, connected in series to the output of the photo-coupler PC1, is regulated at a substantially constant value. The voltage of the resistor R6 is proportional to the output voltage VC1.
The control power supply works in the following manner. A voltage which is N3/N21 times as high as the output voltage VC1 is generated across the tertiary winding N3 of the transformer TR1, i.e., VN3=VC1.multidot.N.sub.T 3/N.sub.T 21, where N.sub.T 3 is the number of turns of the tertiary winding N3 and N.sub.T 21 is the number of turns of the first secondary winding N21 of the transformer TR1. The capacitor C3 is charged via the diode D3 and provides a supply voltage for the control circuits IC1 and IC2. At the same time, DC electric power is fed from the quaternary winding N4 of the transformer TR1 to the capacitor C4 via the diode D4. By equalizing the voltage VC3 of the capacitor C3 and the voltage of the reference diode ZD1 connected to the base of the transistor T4, the electric charges discharged from the capacitor C4 flow to the reference diode ZD1. Since no current flows to the base of the transistor T4, the transistor T4 is off and no electric power is fed to the capacitor C3 through T4.
The power saving operation, which is initiated by the power saving signal applied to terminal 306, is conducted in the following manner. In response to the power saving signal, the transistor T3 turns off and electrically disconnects the resistor R5. Since the control terminal voltage of the shunt regulator IC3 is constant, removing R5 from the circuit reduces the output voltage to terminal 300. The power saving signal also turns on transistors T1 and T2 and the electric power stored in the capacitor C1 is transferred through T1 to the microcomputer power supply composed of the capacitor C2. During ordinary operation, the voltage of the microcomputer power supply is usually lower than the voltage of the main power supply. However, in the power saving mode of operation, the voltage of the main power supply is lowered to the voltage level of the microcomputer power supply.
The power saving signal is also applied to the mode changeover terminals of the control circuits IC1 and IC2 via the photo-coupler PC2. In response to the received signal, the control circuit IC2 is enabled while the control circuit IC1 is disabled, thus placing the circuit in the power saving mode of operation. The control circuit IC2 operates the switching device Q1 at a lower frequency from that in the ordinary operation so that the losses caused in the transformer TR1 and the switch Q1 are reduced.
Finally, the control power supply of the IC's will be explained. The voltage of the tertiary winding N3 also lowers in response to the lowering of the main power supply voltage. The capacitor C4 is fed by the voltage of the quaternary winding N4 through diode D4. When the voltage on the capacitor C3 is less than that of the reference diode ZD1, a current flows from capacitor C4 through the resistor R8 to the base of the transistor T4, thereby turning on transistor T4. As a result, the capacitor C3 receives the electric power for the control circuits IC1, IC2 from the quaternary winding N4 via the diode D4. When the voltage of the capacitor C3 becomes higher than that of the reference diode ZD1, the base current of the transistor T4 no longer flows and the transistor T4 is turned off. As a result, the voltage of the capacitor C3 is equalized to that of the reference diode ZD1. By equalizing the voltage of the tertiary winding N3 to that of the reference diode ZD1, the control voltage, i.e., the voltage of the capacitor C3, is the same in both the ordinary and power saving modes of operations.
In other words, the circuit of FIG. 3 operates to feed the ordinary electric power to the main load at a high frequency that facilitates reducing the dimensions of the transformer and such component parts while, operating at a low frequency to feed the electric power only to the microcomputer during a power saving mode of operation.
However, it is necessary for the circuit of FIG. 3 to use the photo-coupler PC2 for transmitting the changeover signal from the secondary side of the power supply (from the microcomputer) to the control circuits IC1 and IC2 to switch from the ordinary mode of operation to the power saving mode of operation. To provide adequate isolation, it is necessary for the photo-coupler to maintain a certain insulation distance, specified by insulation standards. Therefore, the printed circuit board and such structural parts must have large dimensions, resulting in a large and expensive switching power supply. The use of photo-coupler PC2 for transmitting the changeover signal also increases the number of terminals of the power IC that incorporates the control circuits IC1 and IC2 and the switch Q1, resulting in a large and expensive power IC.